Recently, an illumination apparatus is used that includes a lighting device for lighting a light source unit having groups of semiconductor light emitting elements such as LEDs (Light Emitting Diodes) for emitting lights of different colors including green, blue, red or the like.
In such an illumination apparatus, the chromaticity or intensity of a combined light emitted therefrom can be adjusted by adjusting the intensity of light emitted from each LED group.
There is known a lighting device used in the illumination apparatus, which includes a DC power source circuit and constant current circuits respectively connected in series to LED groups, and in which switching elements included in the respective constant current circuits are controlled by the PWM (Pulse Width Modulation) control to thereby adjust the amounts of lights emitted from the LED groups.
Further, Japanese Unexamined Patent Application Publication No. 2011-258517 (JP2011-258517A) describes a technique in which switching elements are connected to two LED groups of different color temperatures to adjust a duty ratio while alternately turning on and off the LED groups regularly in a time-divisional manner. With this, the amounts of lights emitted from the LED groups are controlled to adjust the color temperature of a light, i.e., a combined light emitted from the two LED groups. According to the technique, it is not necessary to provide a constant current circuit for each LED group, and thus a size of the circuit of lighting device can be smaller and a cost thereof can be reduced.
However, the above-mentioned time-divisional manner, in which the LEDs are lit time-divisionally while adjusting the duty ratio, has problems in that, when a LED group to which a power is supplied is changed sequentially, the switching element may be under stress or an excessive inrush current may flow in a LED group which is turned on by the changing.
That is, since LEDs of different colors have different structures or materials, they have different voltage drops for the same current flow. Therefore, when the LED group to which the power is supplied is changed sequentially, the excessive inrush current may flow in the LED group turned on.
For the LED group in which the excessive inrush current flows, damage to the LED group or its switching element may occur, and the lifespan of the illumination apparatus may be shortened.